Uranium of different isotopic ratios has long been used in research throughout the world to support the nuclear reactor development program. The uranium may be natural, containing the normal ratio of .sup.235 U and .sup.238 U; enriched, where the .sup.235 U content has been raised; or depleted, where the .sup.235 U content has been reduced. In any case, bare uranium as a solid metal will oxidize when exposed to air. The rate of oxidation depends on several factors, including the degree of humidity in the atmosphere; and the oxide builds up on the surface of the parent metal and is radioactive. The oxide is easily brushed or scraped off the parent metal, so much so that anything coming in contact with the uranium becomes contaminated. This then requires an expensive decontamination process.
Finding a suitable protective coating has been a serious and ongoing problem. Many research programs limit the extraneous material that can be introduced into the reactor with the uranium. Hydrogeneous materials are particularly undesirable due to their strong interaction with the neutron flux in the reactor. This precludes the use of most plastics as a coating. Some nonhydrogeneous coatings are available, notably some of the chloro-, fluoro-, bromoethylene polymers. However, this type of coating has a limited useful life (two years or so) whereupon the uranium must be cleaned and recoated. Use and handling can also shorten this life and risk contamination of the area.
Often, the uranium fuel is only between 1/64" and 1/8" thick, and otherwise is shaped as rectangular plates from between 1" and 2" wide and 2" and 4" long. Attempts to encapsulate or clad the uranium plate(s) with stainless steel have not been totally successful either, because even when such encapsulating or cladding structures are formed of thin 15-20 mil (0.015-0.020") sheets, they generally introduce excessive extraneous material to adversely affect the sensitivity of the research. Moreover, manufacture of such structures with the narrow thickness-to-width ratios and with thin sheets of stainless steel has been difficult and unreliable, due to thermal warpage and dimensional instability.
The problem of uranium contamination is even more burdensome in tests involving both uranium and plutonium. Plutonium is almost always cladded or enclosed in a sealed container. Both uranium and plutonium are radioactive, releasing alpha radiation; however, plutonium is also biologically toxic. Thus, preliminary radiation tests are made to detect "leakers" (where the cladding or enclosing container of the plutonium is imperfect), but such tests cannot distinguish between a plutonium container having exterior contamination induced by rubbing against an oxidized uranium fuel plate and a "leaker" container. It then becomes necessary to double check the suspect plutonium container with more costly and sophisticated tests, and it yet further necessitates the decontamination of the container.